There are a few things that may be contributing to this happening. First, with taller people, the blood must travel further to get back to the heart. When the arms are hanging by the side, the blood is more affected by gravity making it harder to get the blood back to the heart (slightly). This can cause some increased pooling of blood in the hands. This is not particularly dangerous since most people don't have their hands hanging completely still for prolonged periods. Any movement will contract the muscles and help "squeeze" the blood back to the heart.
Also, with the hyperthyroidism issue, the vessels may actually be dilating some to help reduce the body temperature and keep it cool. This can cause the blood vessels in the hand to hold more blood and give them the red color you are seeing.
I suspect that in your son's case it is likely a combination of the two. This is not a dangerous thing, only a cosmetic issue.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.